(1) Field of the Invention
The present invention relates to a sheet feed mechanism used to printers and the like and preferable to clamp a sheet by pressing pressure rollers against a sheet feed roller and feed the sheet to a recording section by the rotation of the sheet feed roller.
(2) Description of the Prior Art
In general, serial ink jet printers or thermal transfer printers execute predetermined recording by repeating such operation that a line of data is recorded while moving a carriage on which a recording head is mounted along a platen and thereafter a recording sheet is fed one line for the recording of a next line. A large number of this type of the printers are used as an output device for computers, word processors and the like because they can execute high quality recording in a low-noise environment at a low cost and their maintenance is easy.
FIG. 8 shows a thermal transfer printer as an example of the conventional serial printers. The printer includes a flat-plate-shaped platen 2 disposed at the center of the frame 1 thereof so that the recording surface thereof is located at approximately right angle. A carriage shaft 3 is disposed in parallel with the platen 2 below the platen 2 of the frame 1 in front of it. A flange-shaped guide section 4 is formed to the front edge of the frame 1 and a carriage 5 is mounted on the carriage shaft 3 and the guide section 4 so as to reciprocate along them. A thermal head 6 is mounted on the carriage 5 at the extreme end thereof in confrontation with the platen 2 so that it is caused to come into contact with and be separated from the platen 2 by a not shown drive mechanism. Detachably mounted on the upper surface of the carriage 5 is a ribbon cassette (not shown) which accommodates an ink ribbon in it and guides the ink ribbon between the thermal head 6 and the platen 2.
Further, a winding bobbin 7 for winding the ink ribbon of the ribbon cassette and a feed bobbin 8 on a feed-out side are disposed on the upper surface of the carriage 5, respectively.
A sheet inserting port 9 for inserting a recording sheet (not shown) is formed rearwardly of the platen 2 and further a sheet feed roller 10 for feeding the sheet inserted from the sheet inserting port 9 to the front side (recording section) of the platen 2 is rotatably supported by bearings rearwardly of the platen 2. A plurality of pressure rollers 11 are disposed under the sheet feed roller 10 in parallel therewith so as to come into contact with the surface of the sheet feed roller 10. Each of the pressure rollers 11 has a pressure roller shaft 11a formed thereto so that it is rotatable integrally with the pressure roller 11. Further, the pressure roller shafts 11a are rotatably supported directly by pressure roller holders 11b without using bearings.
On the other hand, a sheet feed gear 12 mounted coaxially with the sheet feed roller 10 projects from a side of the frame 1. The sheet feed gear 12 is connected to the motor gear 15 of a sheet feed motor 14 through a plurality of transmission gears 13, 13. The rotation of the sheet feed motor 14 permits the sheet feed roller 10 to be rotated through the motor gear 15, the respective transmission gears 13 and the sheet feed gear 12 so that the sheet inserted between the sheet feed roller 10 and the pressure rollers 11 from the sheet inserting port 9 is fed while being clamped therebetween.
In the thermal-transfer printer arranged as described above, the sheet is inserted from the sheet inserting port 9, clamped between the sheet feed roller 10 and the pressure rollers 11 and fed to a recording start position in the direction perpendicular to the moving direction of the carriage 5 by the rotation of the sheet feed roller 10 effected by the sheet feed motor 14. Then, desired recording is carried out to the sheet by the partial transfer of ink as a coloring agent to the sheet by selectively energizing the recording elements of the thermal head 6 based on image information while moving the carriage 5 along the platen 2 in the state that the thermal head 6 is pressed against the platen 2 through the ink ribbon and the sheet. Thereafter, each time recording of one line is finished, the sheet is fed for next recording.
Since the above heat-transfer printer is arranged such that each time recording of one line is finished, next recording is executed by feeding the sheet, a pinpoint sheet feed accuracy is required. When a sheet feed accuracy is low and a feed amount of sheet is larger than a predetermined amount, an unrecorded portion or a so-called white stripe is made between lines, whereas when a feed amount of sheet is smaller than the predetermined amount, a double-recorded portion or a so-called black stripe is made to the overlapped portion between lines, and any of the stripes greatly lowers the quality of a recorded image.
Note, sheets used to print include a paper sheet, an OHP sheet composed a transparent resin sheet and the like and they are called a recording sheet or simply a sheet as a whole.
Incidentally, in the conventional sheet feed mechanism, the sheet feed roller 10 is composed of a cylindrical metal core and a rubber roller main body attached, each of the pressure rollers 11 is composed a cylindrical metal core and a rubber roller main body attached to the outer circumference thereof likewise the sheet feed roller 10 and the sheet feed mechanism is composed of the pressure rollers 11 pressed against the sheet feed roller 10.
When each of the sheet feed roller 10 and the pressure rollers 11 is composed of the rubber roller main body attached around the outer circumference of the cylindrical metal core as arranged conventionally, there is a problem that when they are pressed against each other, since the respective roller main bodies of the sheet feed roller 10 and the pressure rollers 11 are elastically deformed and the radii of the respective rollers 10, 11 are changed, a sheet such as an OHP sheet, a paper sheet and the like cannot be fed in an accurate feed amount.
Further, there may be caused a case that the roller main bodies of the sheet feed roller 10 and the pressure rollers 11 are not uniformly deformed in the axial direction thereof and a sheet is obliquely fed.
Further, since the surfaces of both the rollers 10, 11 are formed of rubber, a grip force for clamping a sheet is reduced, by which the sheet is slipped. Thus, the accurate feed amount of the sheet cannot be also obtained in this respect.
Therefore, in the sheet feed roller 10 and the pressure rollers 11 arranged as described above, there is a problem that a white stripe or a black stripe is made between the recorded lines on a sheet, and when overlapped recording such as color recording is carried out, there is caused a problem that recorded positions are dislocated and a recorded image of good quality cannot be obtained.
When the conventional thermal transfer printer can execute heat sublimation print, each of the pressure rollers 11 of the sheet feed mechanism is formed of an elastic resin which does not contain a thermoplastic material because there is a possibility that they are heated to a high temperature. Further, each of the pressure roller shafts 11a is composed of a metal shaft the surface of which is subjected to Ni plating process. Although the pressure roller holders apply pressure contact force to the pressure rollers 11 by urging springs (not shown) disposed therein, the holders are formed of plastics containing highly strong glass fibers to prevent their creep deformation caused by the spring force of the urging springs.
However, the conventional printer sheet feed mechanisms have a problem that when the pressure roller shafts 11a are repeatedly rotated by being directly supported by the pressure roller holders in accordance with the rotation of the pressure rollers 11, the Ni-plated surfaces of the pressure roller shafts 11a are scratched by the glass fibers contained in the pressure roller holders 11b, whereby sliding resistance between the roller shafts and the roller holders is increased and the feed amount of sheet is reduced. For example, the feed amount of an ordinary sheet and a sublimation sheet is reduced by about 0.6-0.8 mm and that of an OHP sheet is reduced by about 1.0-1.4 mm when they are fed 240 mm. Since the reduction of the feed amount On the other hand, sheet eminently appears as a black line when recording is executed by a color printer, improvement is required.